The field of the present invention is braking systems employing antilock devices for vehicles.
Braking systems have been developed which are capable of braking a wheel very efficiently without locking the wheel during braking. Such systems have employed a master cylinder, a wheel brake hydraulically coupled with the master cylinder, a sensor for sensing angular deceleration of the associated wheel capable of specifically sensing the rapid deceleration which would occur at the onset of a locked condition and a modulator in the hydraulic coupling between the master cylinder and the wheel brake which is controlled by the sensor. When the onset of a locked condition is sensed, the sensor causes the modulator to reduce the hydraulic pressure to the brake.
One such braking system is disclosed as an example in Japanese Patent Publication No. 89447/1983 and is presently well known. A modulator of such a device consists generally of an input hydraulic pressure chamber connected to an output port of a master cylinder, an output hydraulic pressure chamber connected to an input port of a wheel brake, a valve port communicating the input and output hydraullic chambers with each other, a valve body capable of opening and closing the valve port, a valve spring urging the valve body to the closed position and a spring biased piston in the output hydraulic chamber for varying the volume thereof. The piston acts to control the valve body and the valve port thereby. the spring on the piston urges the piston in such a way as to reduce the volume of the output hydraulic chamber and to open the valve port. A control hydraulic chamber also opposes pressure in the output hydraulic chamber. A hydraulic pump is connected to the control hydraulic chamber for pressurizing the control hydraulic chamber. A pressure discharge valve is also coupled with the control hydraulic chamber such that it may be opened when the mechanism senses the onset of a locked condition.
In such systems as the one described above, the spring biased piston acting on the output hydraulic chamber will create a residual pressure because of the spring biasing even though the hydraulic pressure is being modulated through pressure discharge in the control hydraulic chamber. Thus, even under conditions of initial wheel locking, hydraulic pressure to the brakes is experienced because of the spring associated with the piston in the output hydraulic chamber. Because of this condition, maximum antilock performance may not be realized on a road surface having a very low coefficient of friction.